A typical operator today may have GSM, WCDMA/HSPA and LTE carriers operating simultaneously on different carrier frequencies. These different Radio Access Technologies, RATs, and corresponding carriers may however have different geographic coverage. For instance, LTE may be deployed in only urban areas, whereas GSM and HSPA coverage may be deployed in both urban and rural regions.
Furthermore, for LTE, more than forty frequency bands are defined in the 3GPP standard, and even if most of them are not universally available frequency bands, an operator in the near future may deploy LTE on multiple carrier frequencies. One or two carriers may be used for coverage and hence deployed in macro cells, while the remaining carriers may be used for hot spot or pico cell coverage. This deployment scenario is especially applicable in urban areas where several LTE carriers on additional frequency layers may be deployed, to provide hot spots in order to cope with high capacity demand.
FIG. 1 illustrates an example in the context of the above scenario. In the diagram, a wireless communication network 10 includes a number of large macro cells 12 that are deployed on a first carrier f0. By way of example, the diagram shows macro cells 12-1 and 12-2, which have at least partially overlapping macro-large-coverage areas. One further sees a number of hotspots or pico cells 14, which individually use one of the carrier frequencies f1, f2, f3 and f4. By way of example, one sees hotspots 14-1 through 14-4 on carrier frequency f1, hotspots 14-5 through 14-8 on carrier frequency f2, hotspots 14-9 through 14-12 on carrier frequency f3, and hotspots 14-13 through 14-15 on carrier frequency f4.
Several of the hotspot carriers may be deployed in the same coverage area. That is, a given hotspot 14 operating on one of the hotspot carriers may overlap geographically with another hotspot operating on another one of the hotspot carriers. For example, there may be overlapping hotspot coverage via carrier f1 and f2 in a given coverage area, while carriers f3 and f4 provide the same or overlapping hotspot service in another coverage area, etc.
For optimal usage of multiple carriers in deployments such as shown in the example of FIG. 1, a wireless communication device operating in the network 10 needs to monitor the carriers based on making inter-frequency measurements. Based on making these inter-frequency measurements, the device reports signal strength for detected cells on respective carriers, to a network, NW, node, such as a supporting base station in the network 10. The NW node then initiates handover, HO, of the device to the then-best carrier and cell, as the serving carrier and cell.
However, typical low-end devices are only equipped with one receiver and hence cannot receive on different carrier frequencies simultaneously. Consequently, such a device needs to interrupt its data reception on a given carrier frequency to perform measurements on other carrier frequencies. Such measurements are performed using configured measurement gaps, which are specified for use in performing measurements on other carrier frequencies. The 3GPP Technical Specification TS 36.300 includes example details regarding measurement gaps, which are periods where the device switches off its receiver and transmitter from a serving cell, so that it can receive transmissions from another cell. These gaps are synchronized with respect to the serving base station of the device, because the serving base station must know when the device will be performing inter-frequency measurements. As is known, Radio Resource Control, RRC, signaling is used to configure the gap period used by the device.
FIG. 2 shows the measurement gap principle as implemented in LTE. A 6 ms gap is allocated every 40 ms or every 80 ms, once inter-frequency measurement gaps are triggered. The 6 ms gap allows time for the device to find synchronization signals and Common Reference Signals, CRS, in the context of inter-frequency LTE measurements, or to find the same kind of signals in the context of inter-RAT measurements, such as where the device makes inter-frequency measurements on WCDMA/HSPA carriers, for example. The gap duration takes switching times into account.
In earlier releases of LTE, inter-frequency measurements in the same RAT or across different RATs was mainly used to address the problem of a device going out of coverage, e.g., going out of a relatively limited LTE coverage area. This problem was more prevalent in the early days of LTE deployment, when LTE coverage was initially quite limited and then expanded over time. For example, an urban area may have LTE coverage along with coverage from one or more other RATs, with the LTE coverage ending at or around the borders of the urban area. In such cases, inter-frequency measurements would be triggered as the device approached the limits of LTE coverage, so that the device began doing inter-frequency measurements and ultimately underwent a handover from LTE to, say GSM or WCDMA, before going out of the LTE coverage. In such contexts, the inter-frequency measurements were only triggered when necessary, and measurement gaps and corresponding inter-frequency measurements were used only when really needed, because measurement gaps reduce the maximum available throughput, and make data scheduling more complex.
For example, a network node responsible for data scheduling needs to take the Hybrid Automatic Repeat reQuest, HARQ, round trip times into account and therefore, using LTE timing as an example, the practical scheduling gap to a device using inter-frequency measurement gaps is ten milliseconds, based on a six millisecond gap time plus a four millisecond HARQ round trip time. This timing translates into a twenty-five percent throughput loss/scheduling time loss, for the case of forty milliseconds between measurement gaps.
In further detail, a device may monitor several frequency carriers, which may be regarded as frequency layers. In Release 11 of the 3GPP specification, depending on the device capability, it may be possible to measure up to seven different frequency layers, including LTE TDD/FDD, WCDMA, GSM, etc. Each frequency layer requires a certain radio time for detection and verification of cells on that layer, and the current 3GPP specification is based on a worst-case scenario with respect to Doppler and delay spread, as well as Signal to Noise Ratio (SNR) requirements on cells on the layer.
Additionally, as discussed above, gap measurement requirements mainly target the coverage problem. Thus, the requirements for inter-frequency measurements are conventionally based on detecting rather weak cells on another carrier frequency, to ensure that a reliable HO can be made prior to going out-of-coverage on the current carrier frequency. For example, with reference to Section 8.1.2.1.1.1 of 3GPP TS 36.133, the current measurement requirements to find a cell is in the order of 3.84*Nfreq seconds, where Nfreq is the number of layers needed to measure on, and where detection is geared towards the detection of a weak signal, e.g., Es/Iot=−4 dB. Consequently, having several layers, as exemplified in FIG. 1, implies that from a specification point of view, the device may need tenths of seconds in gap mode to find a suitable cell for HO. That time is problematic in terms of capacity reduction and other considerations.
In a known mitigation of such problems, a device may be configured to measure only on a subset of frequency layers, e.g., on only two frequency layers among a larger number of available frequency layers. However, this mitigation approach is complicated in a number of respects. For example, the network generally will not know which subset of the frequency layers is most suitable or useful for monitoring by the device. For example, with carriers operating at 2-3 GHz, a difference of only a few meters in device location may change which frequency from among f1, f2 and f3 would be better for the device to camp on.